1. Technical Field
This invention relates to the art of engine lubrication and, more particularly, to oilless lubrication for the piston/cylinder chamber of an uncooled engine.
2. Description of the Prior Art
A low heat rejection engine, particularly for a diesel engine, has the potential to provide significant improvement in fuel economy. Heat rejection can be reduced by eliminating liquid cooling normally incorporated in the block of a diesel engine and replacing all or a portion of the combustion chamber components with materials that can operate at uncooled combustion temperatures, such as ceramics. This is sometimes referred to as an adiabatic diesel engine.
The temperature gradient in such low heat rejection engine will range up to 1600.degree. F. (871.degree. C). At such temperatures, conventional oil, used as a piston lubricant, will pyrolyze. Therefore, some means must be provided to create an antifriction relationship between the cylinder wall and piston which is devoid of fossil lubricants.
One approach, suggested in 1983 by S. Timoney and G. Flynn in an article entitled "A Low Friction, Unlubricated Silicon Carbide Diesel Engine", SAE Paper #830313, was to install a close-fitting SiC piston in a SiC cylinder, the piston having no ring grooves. Blowing of gases past the pistons could not be detected; the authors concluded that the piston must be riding on a gas film due to the reduction in friction horsepower. However, much of their test work was carried out without the engine firing, so a pressurized gas film was not the total reason for nonscuffing but was also due to the low interfacial friction of SiC on SiC. The structure of the Timoney and Flynn piston and cylinder had made no accommodation for thermal growth and assumed uniform dimensions; oil lubrication was fed to the piston pin area which assured little dimensional change and, in fact, contributed to oil lubrication notwithstanding the authors' label of an unlubricated engine. This reference merely defined the problem without providing a specific solution as how to provide a reliable gas phase lubrication while encountering thermal growth, wide variations in the fit, and without oil lubrication. This reference did suggest that if clearances could somehow be controlled, a gas film would function to lubricate the sliding piston in such cylinder.
Thus, it is an object of this invention to be able to control the dimensional clearances between the piston and cylinder of an uncooled oilless internal combustion engine by special selection of materials for anticipated thermal growth and to preshape the cylinder wall and piston to maintain a generally constant gap even under elevated temperatures.